Field of the Invention
This invention relates to plug and a method of manufacturing a spark plug electrode in which an erosion-resistant firing tip is welded to a front end of a composite electrode.
In a spark plug for an internal combustion engine, a firing tip is welded to a front end of a center electrode or a ground electrode.
In order to impart a spark erosion-resistant property with the front end of the center electrode or the ground electrode, it is known that the front end of the electrode is made of nickel-based alloy, while the firing tip is made of a noble metal such as platinum, palladium, iridium and alloys thereof. The firing tip is usually secured to the front end of the center electrode or the ground electrode by means of electrical resistance welding so as to form a dispersion layer at an interface between the firing tip and the front end of the center electrode.
When the electrode is alternately exposed to the heat-and-cool cycle in a combustion chamber of an internal combustion engine, a thermal stress repeatedly occurs at the interface between the firing tip and the front end of the electrode due to the difference of thermal expansion therebetween. The thermal stress is likely to concentrate on the interface to develop cracks so that the firing tip falls off the front end of the electrode with the passage of time while the spark plug is in service.
Therefore, it is an object of the invention to provide an electrode for a spark plug and a method of manufacturing the electrode in which a firing tip is secured to a front end of the electrode by means of laser welding to fuse the firing tip into the front end of the electrode sufficiently, and thus effectively preventing the firing tip from inadvertently falling off the electrode so as to contribute to an extended service life with relatively low cost.